The present invention relates to a method for increasing the braking effect in a motor vehicle with a hydraulic brake system, wherein an energy supply unit, which is used to increase the brake pressure in at least one wheel brake, is switched on when the brake pressure adjusted by the driver in a master brake cylinder exceeds a first activation threshold, which is determined depending on a detected vehicle deceleration, and the energy supply unit is switched off when the brake pressure reaches a predetermined bottom threshold value at a specific instant.
In addition, the invention relates to a device that is suitable for implementing the method.
When subjected to high thermal load, under the influence of saline solution and major wear, hydraulic brakes of a vehicle exhibit a reduced effectiveness so that, even given a high initial pressure in the master brake cylinder of the brake system that is adjusted by the driver, the brake system will not bring about sufficient deceleration of the vehicle. The reduced braking effect at high working temperatures of the brake system is referred to as ‘fading’.
In the cases illustrated hereinabove, the reduced braking effect of the vehicle brake system results, however, especially from a reduced coefficient of friction between the brake pads and the brake disc, so that a special rate of brake pressure is required in order to brake the vehicle effectively.
German published application DE 102 38 427 A1 discloses a method of the type initially referred to, wherein in case of limited effectiveness of the vehicle brake, enabling of an energy source causes increase of the brake pressure or the brake force in the wheel brakes because hydraulic fluid is pumped out of the master brake cylinder into the wheel brakes. A method of this type is also referred to as FBS function (FBS: Fading Brake Support).
The energy source, which is usually a pump, is activated in the FBS function when the initial pressure in the master brake cylinder adjusted by the driver exceeds an activation threshold, which is determined depending on the vehicle deceleration. Thus, the pump is switched on when the actual vehicle deceleration is by a defined value lower than the vehicle deceleration that is expected due to the adjusted initial pressure.
The pump is usually switched off when the initial pressure in the master brake cylinder drops below a predetermined threshold value. This is based on the fact that the braking request of the driver is determined by way of the initial pressure. Thus, it is assumed when the brake pressure declines that the driver releases the brake.
When the energy source is switched on, the initially pressure in the master brake cylinder declines, however, also without the driver releasing the brake because hydraulic fluid is sucked through the pump out of the master brake cylinder.
After deactivation of the energy source, hydraulic fluid flows back into the master brake cylinder, and the initial pressure rises again. When it exceeds the activation threshold again, the FBS function is re-activated.
Thus, a cyclically repeating sequence of activating and deactivating the FBS function occurs during a brake operation, with the frequency at which the cycles repeat being defined by the periodic time variation of the initial pressure.
However, it has been found out that as this occurs, very long phases with a deactivated FBS function generally occur, in which the effectiveness of the brake system is only reduced. This allows achieving only in part the aim of shortening the stopping distance of a vehicle in a case of limited effectiveness of the brake system.
At very high working temperatures of the brake system, overheating of the brake fluid may further cause a drop in pressure in the brake system, which can have as a result that the activation threshold of the FBS function after disabling of the energy source is not reached again. Despite an extremely reduced braking effect, the FBS function will no longer support the brake operation, and the stopping distance of the vehicle becomes considerably longer.
Another reason for the increased volume requirement or drop in pressure in the brake system, respectively, is that the brake pads become ‘soft’ at high temperatures, and that there is, hence, a very low coefficient in friction between the pads and the brake disc.
In view of the above, an object of the invention is to improve the effectiveness of the FBS function and, thus, shorten the stopping distance of the vehicle.